1. Field of the Invention
The present invention relates to a coil energization apparatus of the type that, for example, is used to provide a superconductive coil with electrical current in order to achieve generation of a magnetic field, such as a static magnetic field. The present invention also relates to a method of energizing a superconductive coil of the type that, for example, is used to provide the superconductive coil with electrical current in order to achieve generation of a magnetic field, such as a static magnetic field.
2. Description of the Prior Art
In the field of nuclear Magnetic Resonance Imaging (MRI), a magnetic resonance imaging system typically comprises a superconducting magnet, a gradient coil system, field coils, shim coils and a patient table. The superconducting magnet is provided in order to generate a strong uniform static magnetic field, known as the B0 field, in order to polarize nuclear spins in an object under test.
Presently, it is known to make the coils forming the superconducting magnet from metals that exhibit the property of superconduction at very low temperatures. To achieve superconduction, the superconducting magnet is therefore cooled to the very low temperatures. One known cryogen-cooled superconducting magnet unit includes a cryostat including a cryogen vessel. A cooled superconducting magnet is provided within the cryogen vessel, the cryogen vessel being retained within an outer vacuum chamber (OVC). One or more thermal radiation shields are provided in a vacuum space between the cryogen vessel and the OVC. In some known arrangements, a refrigerator is mounted in a refrigerator sock located in a turret towards the side of the cryostat, the refrigerator being provided for the purpose of maintaining the temperature of a cryogen provided in the cryogen vessel. The refrigerator also serves sometimes to cool one of the radiation shields. The refrigerator can be a two-stage refrigerator, a first cooling stage being thermally linked to the radiation shield in order to provide cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.
During manufacture of the superconducting magnet, at maintenance intervals and/or when installing the superconducting magnet, it is necessary to energize the superconducting magnet to generate the static magnetic field mentioned above, typically using a Direct Current (DC) power supply. The power supply is therefore connected to the magnet coils forming the superconducting magnet via a so-called “Current Lead”. The Current Lead is a pair of conductors, approximately 1 meter long, one end of which is at room temperature while the other is at about 4.2K. The design of the Current Lead is constrained by mutually exclusive properties of the Current Lead: electrical resistance and thermal conductivity. Consequently, design of the superconducting magnet is constrained by limitations associated with the Current Lead in the following ways.
Firstly, the Current Lead limits the maximum current at which the superconducting magnet can operate. In this respect, as the static magnetic field generated by the superconducting magnet is a function of the operating current of the superconducting magnet and/or the number of turns of superconducting wire used to form the coils, it therefore follows that if the superconducting magnet is energized using the maximum current allowed by the Current Lead, the number of turns of superconducting wire used must be relied upon in order to achieve a desired static field strength from the superconducting magnet. Reliance on the number of turns of superconducting wire used is, however, undesirable because the superconducting wire is costly and manufacturing process time is, in part, a function of the number of turns of the coils of the superconducting magnet that has to be provided during manufacture.
Furthermore, when the Current Lead is a so-called fixed Current Lead, i.e. a Current Lead that remains in place after the superconducting magnet has been energized, heat leak is known to occur through the persistent Current Leads. Since the maximum allowable heat leak from the superconducting magnet unit is limited by the refrigeration power of the refrigerator, the more heat leak that occurs through the fixed Current Lead, the less heat leak margin that is available for other parts of the superconducting magnet unit. From a design perspective, the amount of heat leak that can therefore be tolerated from other parts of the superconducting magnet unit, for example suspension components of the superconducting magnet unit, is limited. The use of more thermally efficient materials for the other parts of the superconducting magnet unit is therefore necessitated and this is costly, because whilst, in some cases, the cost of the materials is low, complexity of fitting the materials can be high.
It is also known to use a so-called demountable Current Lead, i.e. a Current Lead that is only connected to the superconducting magnet during charging. An advantage of using the demountable Current Lead is that heat leak is minimized. However, the minimization of the heat leak is at the expense of providing access to a connector arrangement to connect the demountable Current Leads to the superconducting magnet and hence the need to expose the superconducting magnet to atmosphere. Consequently, problems arise when trying to connect the demountable Current Lead to the superconducting magnet due to formation of ice in the connector arrangement provided. Formation of ice within the superconducting magnet unit as a result of the superconducting magnet being exposed to atmosphere is an even greater and particularly costly problem associated with the use of the demountable Current Lead, because the ice formed influences interaction between the coils of the superconductive magnet and a supporting former of the coils resulting in the superconductive magnet being unable to achieve a desired operating field. In more extreme cases, the ice can block the exit of the cryogen with undesirable effects.
Use of High-Temperature Superconductor (HTS) Current Leads has also been suggested as an alternative solution for providing leads to enable the superconducting magnet to be energized, the HTS Current Lead remaining in place after charging the superconducting magnet. In this respect, the cross-sectional area of an HTS Current Lead is less than the respective cross-sectional areas of the conventional fixed and demountable Current Leads described above. Consequently, the HTS Current Leads exhibit less heat leak when in use. However, as the HTS Current Leads are formed from a superconducting material, the use of the HTS Current Leads introduces a risk of the HTS material forming the Current Lead “quenching”, if the current flowing through the HTS Current Leads results in the so-called “critical surface” (characterizing the superconducting material from which the HTS Current Leads are formed) being reached or exceeded. When quenching occurs in one or both of the HTS Current Leads, the energy used to energize the superconducting magnet is dissipated as heat.
In this respect, any heating in the HTS Current Lead (or indeed the non-superconductive leads described above) can result in heating of the superconductive magnet and so part of the superconductive magnet can cease to exhibit the property of superconduction and ohmic heating in the superconducting magnet takes place. The ohmic heating causes so-called “boil-off” of the cryogen used. Boil-off is undesirable as the cryogen is wasted if no recovery system is in place, the cryogen being an expensive commodity.